A cyclic AMP specific phosphodiesterase has been identified in a malignant tumor (P 815) of murine mast cells. The PDE is found primarily (85%) in the soluble fraction of the cell. The enzyme, purified approximately 10-fold by gel filtration, occurs in a single molecular and kinetic form (low Km), and is apparently not dependent on calcium and calmodulin for optimal activity. Although cGMP is hydrolyzed at only 4% of the rate of cAMP hydrolysis, this cyclic nucleotide inhibits cAMP PDE activity by 50-60% at a concentration 2.5 microns. The mast cell PDE activity also displays anomalous behavior on gel filtration and sucrose gradient centrifugation. The Stokes radius was determined by gel filtration to be 54.4 A and the S20,w was determined by gradient centrifugation to be 2.73S. Using these values a molecular weight of 61,000 and a frictional rato of 1.93 were calculated. This PDE is apparently an asymmetric enzyme with novel molecular and regulatory properties.